Yeah, they would fit....the inside parts are the only thing that they changed. The WRX comes with halogen projector lenses. The STI has HID projector lenses, a HID ballast, and the HID bulb with their head lights. Also a electric cervo motor that can slightly adjust the height of the beam pattern. It gets controlled by a little wheel on the left hand side of the dash where the fog lights, I/C sprayer, or HID light height adjustment switches/buttons go.
I'm not sure if it wires right up though or if it would require some electrical changes or not. Otherwise it the STI headlight housing should be the same size.
378awhp/373wtq Custom Dom 3 & CBRD tuned 2006 STI.
"When in doubt...flat out" - Colin McRae